The present invention relates to a method for producing blow moldings, especially to a blow-molding method capable of efficiently producing blow moldings of crystalline thermoplastic resin having good surface gloss and good mold transferability of, for example, embossed patterns and, not warped, having a good appearance with no sinkmarks.
In producing blow moldings of thermoplastic resin, even when a mold having a mirror surface is used, the surface of the blow moldings often have melt fractures and die lines and have pockmarks owing to the gas engulfed and to the crystallization of the resin. In the blow-molding method, therefore, it is impossible to faithfully transfer the surface of the mold used onto the blow moldings produced, and it is difficult to produce blow moldings having a good surface gloss and a good appearance. In particular, the problem is serious in producing blow moldings of crystalline thermoplastic resin.
For producing blow moldings having a good surface gloss, (1) a method has been proposed, which comprises setting a resin parison having, on its surface, a number of small recesses of from 2 to 100 xcexcm in depth, in the cavity of a mold having a mirror surface of at most 0.5S in roughness and having been heated up to a temperature not lower than the crystallization point of the resin, then blow-molding the parison, and thereafter cooling the mold to a temperature not higher than the crystallization point of the resin (Japanese Patent Publication No. 40498/1990). However, the method is problematic in that the parison must be specifically controlled, that the crystalline resin to be molded is limited to one capable of being processed to have melt fractures, and that the melt fractures are unfavorable for improving the appearance of the moldings produced.
(2) Another blow-molding method proposed comprises setting a crystalline resin-containing flexible resin parison in the cavity of a mold, clamping the molds, applying a pressure fluid into the parison to thereby make the parison airtightly pressed against the surface of the clamped mold, and cooling the thus-molded parison, in which the temperature of the mold is kept between a temperature around which the crystallization rate of the crystalline resin is the largest and the melting point of the resin, a coolant fluid is introduced into the parison under pressure, and the fluid is circulated still under pressure (Japanese Patent Laid-Open No. 77231/1992).
In this method, die lines and weld lines that may appear in the surface of the moldings produced could be reduced. However, the method is still problematic in that the surface of the moldings taken out of the mold is often roughened and stained. In addition, in the method in which a coolant medium is circulated, the coolant efficiency is not all the time good. Therefore, shortening the molding cycle of the method is naturally limited, and the method is not suitable to producing large-size blow moldings.
(3) In still another method proposed, used is a resin having a low melt viscosity. However, the method is problematic in that the resin drawdown is great and the method is not applicable to producing large-size moldings. (4) In still another method proposed, used is a resin of good transferability for an outer layer of moldings. However, the method could not be a substantial solution of the problem, since the resin to be molded therein is limited and since the method requires a complicated apparatus equipped with a multi-layer die.
(5) In still another method proposed, a thermoplastic resin is molded in a mold while the surface of the mold is heated up to a temperature not lower than the Vicat softening point of the resin, and then it is cooled to a temperature not higher than (the Vicat softening point xe2x88x9210xc2x0 C.) (Japanese Patent Laid-Open No. 276432/1996). The method is favorable to amorphous thermoplastic resins such as ABS resins, but is not to crystalline resins such as polypropylene resins.
(6) In still another method proposed for blow-molding polypropylene, the mold is at least temporarily heated up to a temperature not lower than 140xc2x0 C., after the start of clamping the mold and before taking the blow molding out of the mold (Japanese Patent Laid-Open No. 138324/1998). The method is suitable for producing blow moldings having a surface gloss of at least 75%, and the blow moldings produced are for exterior parts of automobiles. However, the method generally requires a step of blow-molding the resin at a mold temperature not higher than 80xc2x0 C. followed by a step of heating the resulting blow molding up to a temperature not lower than 140xc2x0 C., as is obvious from its examples demonstrated in the laid-open publication. In this method, therefore, the temperature range for heating and cooling the mold in one molding cycle will be broad.
The technique of blow-molding is suitable for producing lightweight, large-size moldings, since the molding pressure is low, the mold and its material are inexpensive and the mold-clamping pressure is low. However, since the molding pressure is low, the mold transferability onto the moldings is not satisfactory. For improving it, it has been proposed to elevate the temperature of the mold surface while the parison in the mold is blow-molded. For example, for crystalline thermoplastic resins, the mold surface is heated up to a temperature around the crystallization point of the resin; and for amorphous thermoplastic resins, it is heated up to a temperature around the Tg of the resin. However, such an increased mold surface temperature is problematic in that the moldings formed are difficult to cool and therefore the molding cycle is inevitably prolonged and the productivity is thereby lowered.
For cooling the moldings, the elevated mold temperature is lowered in every molding cycle. Apart from this, various other methods have been proposed. One method proposed comprises blow-molding a thermoplastic resin parison in a mold followed by introducing a coolant medium into the resulting molding to thereby directly cool it. Anyhow, for cooling the blown parison in the blow-molding method, the mold is cooled, or a coolant medium is introduced into the blown parison to directly cool it, or the two are combined. In the blow-molding method, therefore, the matter of great importance is how to efficiently cool the blown parison. Various proposals have heretofore been made for increasing the cooling rate of blown parisons.
With the development of the technique of blow-molding, extremely complicated blow moldings and large-size blow moldings have come available. In particular, blow moldings of crystalline thermoplastic resin such as polypropylene resin are being much used these days for automobile parts such as bumpers and air spoilers, for housings for musical instruments, and for containers, as they are lightweight and recyclable and can be entirely made of one and the same type of resin. It is important that the moldings for them should have a good appearance, and mirror surface transfer of the mold used to the moldings produced must be ensured. For such mirror surface transfer, the mold temperature at which a resin parison is blow-molded into a molding must be near the crystallization point of the resin. Accordingly, after a resin parison has been blow-molded in a high-temperature mold, the resulting blow molding is cooled to have a good surface gloss.
In a process of blow-molding crystalline thermoplastic resin, the blow moldings produced must be cooled at a constant cooling rate. If not, or that is, if the blow moldings are not cooled uniformly, they will have sinkmarks and will be warped, and, in addition, their dimensional accuracy will lower. As a result, they have no market value, and will be impracticable. For uniformly cooling the blow moldings, the cooling rate may be lowered. In ordinary blow-molding, however, the mold temperature is generally kept near the crystallization point of the resin being blow-molded. Therefore, if the cooling rate is lowered in the method, it will be often impossible to substantially cool the blow moldings at such a low cooling rate. Even if they could be cooled at such a low cooling rate, the cooling time will be prolonged, and this will be against the improvement of productivity that shall be attained by reducing the molding cycle.
The object of the present invention is to provide a method for efficiently producing blow moldings of crystalline thermoplastic resin, in which the blow moldings produced have good mold transferability and good surface gloss and, not warped, have good dimensional stability with no sinkmarks.
We, the present inventors have assiduously studied blow moldings of crystalline thermoplastic resin, discussing the problem of how to improve their surface gloss and their mold transferability of, for example, embossed patterns and how to prevent the blow moldings from having sinkmarks and from being warped. As a result, we have found that, when the cooling condition in the method of producing the blow moldings is controlled, then the blow moldings produced have a good surface gloss and, not warped, have a totally excellent appearance with no sinkmarks. On the basis of this finding, we have completed the present invention.
Specifically, the invention provides the following:
(1) A method for producing blow moldings of crystalline thermoplastic resin, which comprises setting a melt parison of a crystalline thermoplastic resin in a mold and then blowing the parison, and is characterized in that the parison is blown and shaped at a mold temperature falling between (the crystallization point of the resin xe2x88x9210xc2x0 C.) and (the melting point of the resin), and then cooled, and that the blown parison is, after kept at a temperature falling between (the crystallization point of the resin xe2x88x9215xc2x0 C.) and (the crystallization point thereof xe2x88x9245xc2x0 C.) for a predetermined dwell time, cooled in an ordinary manner.
(2) The blow-molding method of above (1), wherein the dwell time falls between 30 and 300 seconds.
(3) The blow-molding method of above (1), wherein the mold temperature falls between (the crystallization point of the resin xe2x88x9210xc2x0 C.) and (the crystallization point thereof +10xc2x0 C.), ad the dwell time falls between 40 and 250 seconds.
(4) The blow-molding method of any of above (1) to (3), wherein the mold cooling rate falls between 50 and 500xc2x0 C./min.